I could see the steering rack as a potential fouling point - mostly because the vast majority of Supra parts are made in the US, and therefore LHD (and may not account for the steering rack on RHD). As to the GE turbo vs GTE question, this is exactly the same as in A80 Supras (one of which I own): cheaper to turbo the GE than replace with a GTE, but only as long as you don't want a serious power hike - because then you'll start breaking gearboxes and differentials, which make it cost effective to have swapped the entire driveline to the Turbo model. Not to mention the cost of a full standalone ECU, gauges, additional coolers etc.
Mostly, I just wanted to confirm that if I got an IS300 (which I want) and wanted more power (which I think is probably inevitable for me), I am in the same situation as if I owned a non-turbo Supra.